The present invention relates to a drive voltage generating apparatus, which includes drive voltage generating sections that perform switching of a direct-current voltage and output the voltage to load devices. The present invention also pertains to a method for controlling the apparatus.
Inverter circuits in a motor controlling apparatus and a step-up circuit and a step-down circuit, which are used as power supply, perform switching of a direct-current voltage and output the voltage to load devices. (Hereinafter, step-up circuits and step-down circuits, which function as a power supply, and motor controlling apparatuses are referred to as drive voltage generating apparatuses.)
An inverter circuit in a motor controlling apparatus generates a drive voltage (PWM pulse) by pulse width modulation (PWM) control, and supplies the generated voltage to coils of a motor, thereby driving the motor. For example, an inverter circuit in a motor controlling apparatus for a three-phase direct-current motor has three pairs of switching elements. Each pair of the switching elements corresponds to one of U-phase coil, V-phase coil, and W-phase coil. One of the switching elements in each pair is connected to a positive electrode of a direct-current power supply, and the other switching element in the pair is connected to a negative electrode of the power supply. Each pair of the switching elements supplies drive voltage to the corresponding one of the U-phase coil, the V-phase coil, and the W-phase coil. As the switching elements, power transistors, power field transistors (power FETs), or insulated gate bipolar transistors (IGBT) are used.
A typical motor controlling apparatus samples analog signals representing the values of currents through coils of a motor or analogue signals representing output torque of the motor, thereby converting the signals into digital data. The motor controlling apparatus controls the motor based on the obtained digital data.
In the PWM control, as shown in FIG. 9, a triangular wave WA3 the period of which is predetermined is compared to a comparison signal WB3 that is in inverse proportion to a voltage supplied to the motor. The switching elements are turned on and off at times where the triangular wave WA3 and the comparison signal WB3 intersect each other. Waveforms WUa3 and WUb3 represent ON/OFF states of a pair of switching elements. WUa3 and WUb3 correspond to one phase (U phase) of the inverter circuit. In FIG. 9, WUa3 represents the ON/OFF state of a switching element connected to a positive electrode of a direct-current power supply, and WUb3 represents the ON/OFF state of a switching element connected to a negative electrode of the power supply. When WUa3 is on and WUb3 is off, a U-phase coil of the motor receives the voltage (positive voltage) of the positive terminal of the direct-current power supply. In the reverse state, the U-phase coil receives the voltage (negative voltage) of the negative terminal of the direct current power supply. Therefore, when the comparison signal WB3 is relatively close to a vertex A of a crest of the triangular wave WA3, the positive voltage is outputted only for a short period of time. This lowers the average output voltage in a single control period. On the other hand, when the comparison signal WB3 is in the vicinity of a vertex B of a trough of the triangular wave WA3, the positive voltage is outputted for an extended period of time. This raises the average output voltage in a single control period. In this manner, the ON period and the OFF period of the switching elements in a single control period are controlled. Accordingly, the average output voltage supplied to the motor is varied in a single control period. The PWM pulse is smoothed by the reactance of the coils of the motor. Accordingly, a near-sinusoidal current waveform is formed. That is, by controlling the switching of the switching elements, currents through the coils of the motor are controlled. The torque and the speed of the motor are controlled, accordingly.
A switching element generates electromagnetic waves when performing switching. As a result, switching noise is mixed with signals through analog signal lines. If noise is superimposed on an analog signal representing a current through a coil (for example, a signal WV3 in FIG. 9) and the signal is sampled in this state, the resultant digital data will be erroneous. As a result, accurate control cannot be carried out.
Conventionally, the following measures are taken against the above drawbacks.
(1) The time constant of an analog signal is increased by providing a capacitor in an analog signal line.
(2) Sampling of the analog signal is performed two or more consecutive times. When it is determined that noise has been superimposed on a signal during the sampling, sampling is started over.
(3) Noise-proof lines such as shielded twisted-pair wires are used to transmit analog signals from sensors to an area in the vicinity of an analog-digital converter, thereby preventing noise from being mixed with analog signals.
(4) A value representing an output current of an inverter is differentiated and then multiplied with the impedance. The resultant is used to indirectly detect the load voltage of a load that is driven by the inverter. This method is disclosed in Japanese Laid-Open Patent Publication No. 2002-247856.
Japanese Laid-Open Patent Publications No. 11-299229 and No. 2002-101647 disclose step-up circuits and step-down circuits. These circuits perform switching of a direct-current voltage with switching elements to increase or decrease an input voltage. Therefore, for example, in a case where an input voltage, an output voltage, or a load current is detected and feedback controlled, the above described problems occur. When a step-up circuit or a step-down circuit is used with a motor controlling apparatus, noise due to switching in the step-up circuit or the step-down circuit can be superimposed on analog signals in the motor controlling apparatus.
If the method (1) is applied, the waveform of an analog signal becomes dull. Particularly, when the analog signal changes at a high frequency, accurate information cannot be obtained.
If the method (2) is applied, load on a central processing unit is increased due to the process for multiple sampling and the determination on whether noise is superimposed. Also, since the timing of sampling in a single period varies between a case where noise is superimposed and a case where no noise is superimposed, the accuracy of the control cannot be improved.
If the method (3) is applied, complex wiring must be manually completed during manufacture of the controller. Also, the costs of materials increase the total cost.
If the method (4) is applied by using hardware to perform processes such as differentiation, circuits for differentiation increases the costs. If the differentiation is performed the central processing unit, load on the central processing unit is increased. This can reduce the amount of time that the central control unit spends for controlling. Also, since the computation takes time, the realtimeness of detected values can be spoiled.